Re: 100% iowait on one of cpus in current -git

[Torsten Kaiser]

On 11/1/07, Fengguang Wu <wfg@xxxxxxxxxxxxxxxx> wrote:
> On Wed, Oct 31, 2007 at 04:22:10PM +0100, Torsten Kaiser wrote:
> > Since 2.6.23-mm1 I also experience strange hangs during heavy writeouts.
> > Each time I noticed this I was using emerge (package util from the
> > gentoo distribution) to install/upgrade a package. The last step,
> > where this hang occurred, is moving the prepared files from a tmpfs
> > partion to the main xfs filesystem.
> > The hangs where not fatal, after a few second everything resumed
> > normal, so I was not able to capture a good image of what was
> > happening.
>
> Thank you for the detailed report.
>
> How severe was the hangs? Only writeouts stalled, all apps stalled, or
> cannot type and run new commands?

Only writeout stalled. The emerge that was moving the files hung, but
everything else worked normaly.
I was able to run new commands, like coping the /proc/meminfo.

[snip]
> > After this SysRq+W writeback resumed again. Possible that writing
> > above into the syslog triggered that.
>
> Maybe. Are the log files on another disk/partition?

No, everything was going to /

What might be interesting is, that doing cat /proc/meminfo
>~/stall/meminfo did not resume the writeback. So there might some
threshold that only was broken with the additional write from
syslog-ng. Or syslog-ng does some flushing, I dont now. (I'm using the
syslog-ng package from gentoo:
http://www.balabit.com/products/syslog_ng/ , version 2.0.5)

> > The source tmpfs is mounted with any special parameters, but the
> > target xfs filesystem resides on a dm-crypt device that is on top a 3
> > disk RAID5 md.
> > During the hang all CPUs where idle.
>
> No iowaits? ;-)

No, I have a KSysGuard in my taskbar that showed no activity at all.

OK, the subject does not match for my case, but there was also a tmpfs
involved. And I found no thread with stalls on xfs. :-)

> > The system is x86_64 with CONFIG_NO_HZ=y, but was still receiving ~330
> > interrupts per second because of the bttv driver. (But I was not using
> > that device at this time.)
> >
> > I'm willing to test patches or more provide more information, but lack
> > a good testcase to trigger this on demand.
>
> Thank you. Maybe we can start by the applied debug patch :-)

Will applied it and try to recreate this.

Thanks for looking into it.

Torsten
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